How should one synchronize customized kernel patches?

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I maintain a customized kernel, also I provide patches that you can use directly for major version of kernel source code, now I got a classic problem,
I developed the patch based on 3.7.8 kernel, as newer version 3.7.9 is out, I had to develop the patch for 3.7.9 as well, but how should I do this quickly?
Right now, I download the whole source code of 3.7.9, migrate the code to new version of kernel manually, and generate the patch from newer kernel source. That looked stupid and inefficient.
------------------- A --------------------
| My 3.7.8 kernel | ---> | My 3
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